Sheet-breakup characteristics of a closed-type swirl injector considering internal flow instability

Abstract Sheet-breakup characteristics of a single liquid swirl injector is important for understanding swirling flow characteristics of the swirl coaxial injector. Also, acquirement of a precise breakup length of a swirl injector could be used as a guideline for designing the injector for small thrusters. Theories for obtaining this breakup characteristic have been developed using the waveform instability of a thin liquid sheet. However, these theories could not explain the effect of internal instability of the swirl injector on the breakup characteristic. In the swirl injector, internal instability could be caused by air core fluctuation and this instability could affect the wave and breakup characteristics of the spray sheet. Therefore, in this study, the effect of internal flow instability of the swirl injector on the spray-sheet breakup characteristics was investigated. At first, it was proposed to substitute the new value of the internal growth rate ωint for the existing growth rate, and an empirical formula for the internal growth rate value was presented based on experimental results. Through this equation, it was confirmed that the breakup length could be more accurately calculated using the swirl injector condition in which the internal flow instability of the injector was the dominant influence. Then, a semi-empirical equation for calculating the breakup length more accurately was proposed.